Move roles/matrix* to roles/custom/matrix*

This paves the way for installing other roles into `roles/galaxy` using `ansible-galaxy`,
similar to how it's done in:

- https://github.com/spantaleev/gitea-docker-ansible-deploy
- https://github.com/spantaleev/nextcloud-docker-ansible-deploy

In the near future, we'll be removing a lot of the shared role code from here
and using upstream roles for it. Some of the core `matrix-*` roles have
already been extracted out into other reusable roles:

- https://github.com/devture/com.devture.ansible.role.postgres
- https://github.com/devture/com.devture.ansible.role.systemd_docker_base
- https://github.com/devture/com.devture.ansible.role.timesync
- https://github.com/devture/com.devture.ansible.role.vars_preserver
- https://github.com/devture/com.devture.ansible.role.playbook_runtime_messages
- https://github.com/devture/com.devture.ansible.role.playbook_help

We just need to migrate to those.

docs/configuring-playbook-bridge-appservice-kakaotalk.md

@@ -28,8 +28,8 @@ There are some additional things you may wish to configure about the bridge.
 
 Take a look at:
 
-- `roles/matrix-bridge-appservice-kakaotalk/defaults/main.yml` for some variables that you can customize via your `vars.yml` file
-- `roles/matrix-bridge-appservice-kakaotalk/templates/config.yaml.j2` for the bridge's default configuration. You can override settings (even those that don't have dedicated playbook variables) using the `matrix_appservice_kakaotalk_configuration_extension_yaml` variable
+- `roles/custom/matrix-bridge-appservice-kakaotalk/defaults/main.yml` for some variables that you can customize via your `vars.yml` file
+- `roles/custom/matrix-bridge-appservice-kakaotalk/templates/config.yaml.j2` for the bridge's default configuration. You can override settings (even those that don't have dedicated playbook variables) using the `matrix_appservice_kakaotalk_configuration_extension_yaml` variable
 
 
 ### Set up Double Puppeting